Present commercial rechargeable batteries use graphitic carbons as the anodic active material. They exhibit good dimensional stability, stable cycling performance, are relatively cheap and readily available. Their intercalation chemistry and reactivity towards lithium is well understood. Graphite can intercalate one lithium per six carbon atoms, limiting its theoretical specific capacity to 372 Ah/kg. This capacity however does gradually drop with cycling resulting from the decomposition reactions of carbon with the electrolyte [1,2] and/or the delamination of the graphite. Li4Ti5O12 is another widely studied intercalation material due to the minimal changes that occur in its unit cell upon insertion of up to one lithium per formula unit. Small structural hysteresis coupled with rapid lithium diffusion makes this material electrochemically interesting [3]. However, a low electronic conductivity which leads to an initial capacity loss is the main obstacle preventing its commercial application [4].
Although Li based alloys (e.g. LixM, M=Al, Si, Ge, Sn, Pb, and Bi) show very high specific capacity, volume changes upon lithiation and delithiation induce high mechanical stress followed by pulverization of the electrode [5,6]. A good example is aluminium which reacts with lithium at 0.3 V vs. Li/Li+ to form LiAl. Even though both Al and LiAl have a fcc (face centered cubic) structure, aluminium atoms no longer occupy the same positions because the grand crystal structure changes. This phase transition is accompanied by a 200% volume expansion compared to pure aluminium and hence the aluminium electrodes fail to maintain their structural integrity upon electrochemical cycling. Intermetallic insertion hosts such as Cu6Sn5 [6-8] show reasonable gravimetric and volumetric capacities without any excessive dimensional changes. In the case of Cu6Sn5, the electrochemical reaction with lithium induces a phase transition from the nickel arsenide type structure to the zinc blende type structure. However, the displacement of large tin atoms during the structural change slackens the kinetics and induces hysteresis during cycling.
Nanostructured Fe2O3 [9], Fe3O4 [10], Co3O4 [11] were shown to have great potential as negative electrode materials. In their cases the electrochemical reactions proceed by a complete reduction of the metal oxide to the metallic particles, referred to as a ‘conversion reaction’. Different transition metal fluorides (FeF3, CoF3) [12] and phosphides (CoP3, NiP2, VP4) [13-15] undergo conversion reactions with lithium resulting in high specific capacities. However, these reactions fundamentally suffer from poor kinetics that leads to a large polarization (i.e. a hysteresis between the charge and the discharge voltage), poor capacity retention on cycling, irreversible capacity loss in the first cycle and low columbic efficiency.
Another promising class of materials are metal nitrides. Metal nitrides, are interesting materials because they have a high melting point, are chemically inert and resistant to moisture and erosive environments. Additionally, in comparison to oxides transition metal nitrides show a lower intercalation potential, due to the lower electronegativity and larger polorizability of nitrogen.
Some ternary lithium nitrides crystallise with a filled-up antifluorite structure (Li2x-1MNx (M=Fe, Mn, V)) or with a layered Li3N (Li3-xMxN (M=Co, Ni, Cu)) type structure. Lithium extraction from these materials induces the formation of an amorphous phase LiMxN. The antifluorite systems Li7MnN4 and Li3FeN2 have interesting electrochemical behaviour although their specific capacities are poor (about 250 Ah/kg) [20-22] and remain unstable. In their cases the deintercalation of lithium results in the formation of new undefined phases with lower lithium content [22, 24]. Amongst all studied ternary nitrides, Li2.6Co0.4N exhibits the best stability and a large capacity of 700 Ah/kg [16-19]. These results however remain questionable because electroneutrality demands a switch from Co1+ to Co4.25+ on going from Li2.6Co0.4N to Li1.3Co0.4N corresponding to the capacity claim of 700 Ah/kg, which is highly unrealistic with a nitrogen coordination.
However, a major limitation of many nitrides is their spontaneous decomposition under ambient conditions by reaction with water and oxygen. They react with water by means of an acid-base reaction releasing ammonia and conversion of the nitrides to oxides and hydroxides. Their interaction with oxygen may replace nitrogen by a redox reaction leading to the formation of gaseous nitrogen along with other oxides [24].
The use of binary nitrides like silicon oxy nitride, Si3N4 [25], InN [26], Zn3N2 [27], Cu3N [28], Co3N [29], Fe3N [29], Ge3N4 [30] was studied as a possible solution because of their enhanced stability and because they do not require any prior delithiation. All of these materials have been reported to undergo irreversible conversion reactions with lithium forming amorphous Li3N matrices and metal nanoparticles as shown in the equation [31]:MxNy+3yLi++3ye−→xM+yLi3N  (1)
Upon electrochemical reaction with lithium Zn3N2 exhibits a large reduction capacity of 1325 Ah/kg corresponding to the insertion of 3.7 Li per Zn atom. This capacity however decreases within a few cycles until it tends to stabilize around 550 Ah/kg. The formation of LiZnN as the new end member of the electrochemical reaction with lithium was identified as the cause of the irreversible loss observed during the first cycle. Very poor cycle life limits the prospect of this material. Cu3N exhibits good cycling efficiency but the copper nitride conversion process is sluggish and the voltage profile is not optimal for commercial applications [28]. In spite of a high capacity of 500 Ah/kg and good cycling stability, the use of Ge3N4 is prohibitive because of its high costs [30].
Amongst various binary nitrides, vanadium nitride (VN) is an interesting material because of its high electronic conductivity as well as mechanical and thermodynamical stability. Vanadium is a widely available, cheap and non toxic metal. Additionally, the nitrides of vanadium in comparison to its oxides exhibit good electronic conductivity. Superconductivity of VN and its dependence on V/N stoichiometric ratio has been studied extensively [32]. Recently VN has been shown to act as a supercapacitor delivering an impressive specific capacitance of 130 F g−1 [36].
Vanadium nitride thin films have been investigated as anode material for future rechargeable lithium batteries [37]. Such films were prepared by magnetron sputtering and they were shown to decompose to V metal after discharging to 0.01 V [37].